12824
J. Am. Chem. Soc. 1997, 119, 12824-12830
Novel Quaternary Metal-Rich Phosphides: Stabilization by Differential Fractional Site Occupancies and Polar Intermetallic Bonding Holger Kleinke† and Hugo F. Franzen* Contribution from the Ames Laboratory, DOEsIowa State UniVersity, Ames, Iowa 50011 ReceiVed June 6, 1997X
Abstract: The new phosphide Hf5Nb5Ni3P5 has been prepared by arc-melting of a pressed stoichiometric mixture of HfP, Nb, and Ni. Single crystals suitable for the structure determination were obtained after annealing in an induction furnace at 1350 °C. Hf5Nb5Ni3P5 crystallizes in the hexagonal space group P6h2m (No. 189). The early transition metal atoms Hf and Nb are found on three crystallographically different sites, with refined occupancies of 100% Hf for the M1, 59.5(4)% Hf and 40.5(4)% Nb for the M2, and 13.1(6)% Hf and 86.9(6)% Nb for the M3 site. Small variations of the Hf to Nb ratio are possible, occurring with significantly different lattice parameters. The early transition elements form a three-dimensional framework with numerous M-M (M ) Hf, Nb) interactions, including Ni and P in its trigonal prismatic voids. The differential fractional site occupancies can be understood on the basis of the different preferences of Hf and Nb to form M-M, M-Ni, and M-P bonds. The metallic character is confirmed by the Pauli paramagnetism experimentally obtained.
Introduction The model of differential fractional site occupancies (DFSO) was first used to explain the occurrence of new structure types in the quasibinary system Nb-Ta-S which are unknown among the binary sulfides. The four sulfides Nb1.72Ta3.28S2,1 Nb0.95Ta1.05S2,2 Nb4.92Ta6.08S4,3 and Nb6.74Ta5.26S44 all have no counterpart among the binaries, although Nb and Ta are closely related metal atoms. The three criteria for DFSO stabilized materials are as follows: (i) each metal atom site is statistically occupied by two different metal atoms, (ii) the fractional site occupancies per site are more or less fixed, but differ from one site to another, and (iii) the structures of the DFSO materials are unknown in the binary systems. The similarity of Nb and Ta enables mixed Nb/Ta occupancies on each metal site in these sulfides, but the metals are apparently different enough to show clear preferences for the different metal sites. It has been shown by calculating the Pauling bond orders (PBO) and the Mulliken overlap populations (MOP), using the extended Hu¨ckel approximation, that the Nb/Ta ratio increases with decreasing metal-metal interactions.5,6 The preference of Ta for the sites with more metalmetal interactions is based on the greater extension of its 5d orbitals, compared to the Nb 4d orbitals. The DFSO concept is not limited to sulfides of metal pairs of the same group. Recently, two new structure types have been uncovered with Zr6.45Nb4.55P47 and Hf5.08Mo0.92P3.8 In both cases, the more electron rich metal atom (Nb and Mo, † Present address: Philipps-Universita ¨ t Marburg, Fb Chemie, D-35032 Marburg, Germany. X Abstract published in AdVance ACS Abstracts, December 15, 1997. (1) Yao, X.; Franzen, H. F. J. Am. Chem. Soc. 1991, 113, 1426-1427. (2) Yao, X.; Miller, G. J.; Franzen, H. F. J. Alloys Compd. 1992, 183, 7-17. (3) Yao, X.; Franzen, H. F. J. Solid State Chem. 1990, 86, 88-93. (4) Yao, X.; Franzen, H. F. Z. Anorg. Allg. Chem. 1991, 598/599, 353362. (5) Yao, X.; Marking, G. A.; Franzen, H. F. Ber. Bunsenges. Phys. Chem. 1992, 96, 1552-1557. (6) Franzen, H. F.; Ko¨ckerling, M. Prog. Solid State Chem. 1995, 23, 265-289. (7) Marking, G. A.; Franzen, H. F. Chem. Mater. 1993, 5, 678-680. (8) Cheng, J.; Franzen, H. F. J. Solid State Chem. 1996, 121, 362-371.
S0002-7863(97)01868-4 CCC: $14.00
respectively) prefers the sites with higher metal-metal bond order. On the other hand, the isostructural ZrNbP and HfNbP (Co2Si type) show complete ordering on the metal sites.9 To date, the DFSO concept has only been applied in quasibinary systems. Expanding the DFSO concept to quasiternary systems seemed to us to be promising, because several ternary pnictides and chalcogenides with new structure types and interesting physical and structural properties have been uncovered in the past. The ternaries, consisting of an early and a late transition metal atom and a nonmetal atom, are stabilized by strong earlylate transition metal bonds, which correspond to Lewis’ acid/ base concept. The structures of Nb9Ni2-xS3+x,10 Ta9M′2S6 (M′ ) Fe, Co, Ni),11,12 Ta11M′2Se8 (M′ ) Fe, Co, Ni),13 Ta8NiSe8,14 Zr6M′Te2 (M′ ) Mn, Fe, Co, Ni, Ru, Pt),15 Hf8M′Te6 (M′ ) Mn, Fe),16 Hf5M′Te3 (M′ ) Fe, Co),17 M2M′P,18,19 Hf5Co1+xP3-x,20 Hf5Ni1+xP3,21 Zr9M′2P4 (M′ ) Co, M),22 and Zr6CoAs223 contain extended 3D networks of the early transition element with late transition elements as interstitials, located in one-, two-, or three-capped trigonal prisms. Trying to synthesize quaternary phosphides with two different early transition elements M and one late transition element M′ is assumed to substantially complicate the DFSO concept. In such a case, one has to consider three different trends, i.e. for the M-M, M-M′, and M-P interactions. Our first attempts yielded the novel Hf4.96(4)Nb5.04(4)Ni3P5, which forms a new (9) Marking, G. A.; Franzen, H. F. J. Alloys Compd. 1994, 204, L17L20. (10) Harbrecht, B. Z. Kristallogr. 1988, 182, 118-120. (11) Harbrecht, B.; Franzen, H. F. J. Less-Common Met. 1985, 113, 349360. (12) Harbrecht, B. J. Less-Common Met. 1986, 124, 125-134. (13) Harbrecht, B. J. Less-Common Met. 1988, 141, 59-71. (14) Conrad, M.; Harbrecht, B. J. Alloys Compd. 1993, 197, 57-64. (15) Wang, C.; Hughbanks, T. Inorg. Chem. 1996, 35, 6987-6994. (16) Abdon, R. L.; Hughbanks, T. Chem. Mater. 1994, 6, 424-428. (17) Abdon, R. L.; Hughbanks, T. J. Am. Chem. Soc. 1995, 117, 10035. (18) Kleinke, H.; Franzen, H. F. Angew Chem., Int. Ed. Engl. 1997, 36, 513-516. (19) Kleinke, H.; Franzen, H. F. J. Solid State Chem. 1997, 131, 379386. (20) Kleinke, H.; Franzen, H. F. J. Alloys Compd. 1996, 229, 40-53. (21) Kleinke, H.; Franzen, H. F. Chem. Mater. 1997, 9, 1030-1035. (22) Kleinke, H.; Franzen, H. F. Inorg. Chem. 1996, 35, 5272-5277. (23) Kleinke, H. J. Alloys Compd. 1997, 252, L29-L31.
© 1997 American Chemical Society
NoVel Quaternary Metal-Rich Phosphides
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12825
Table 1. Cell Dimensions of HfxNb10-xNi3P5 with Varying x Parameter x
a/Å
c/Å
V/Å3
4a 5b 5a 6a
9.5556(3) 9.561(4) 9.5698(7) 9.5870(4)
3.5009(2) 3.5237(5) 3.5250(4) 3.5373(3)
276.84(2) 279.0(2) 279.57(5) 281.56(3)
a Cell dimensions obtained by indexing the powder diagram of the bulk sample. b Cell dimensions obtained from single crystal data.
structure type with three different M sites. All three M sites have short contacts to Ni as well as P sites. To our knowledge this is the first example of a fractional Hf/Nb occupation on different atom sites. We report here the synthesis, phase range, crystal and electronic structure, and physical properties. Calculations of the different bond orders and overlap populations help in understanding the different fractional site occupancies in this case also, proving the possibility of a more general use of the DFSO concept. Experimental Procedures Syntheses. The monophosphides HfP and NbP were prepared in sealed silica tubes at a maximum temperature of 800 °C, starting from the elements in the stoichiometric ratio (Hf: ALFA AESAR, powder, -325 mesh, purity 99.6% (containing 2-3.5% Zr); Nb: ALFA AESAR, powder, -60 mesh, purity 99.8%; P: ALFA AESAR, powder, -100 mesh, red amorphous, 99%). To avoid a high vapor pressure of elemental phosphorus, the samples were annealed at 500 °C over a period of 1 week. Thereafter, all phosphorus has reacted to form Hf and Nb phosphides, respectively. The tubes were then heated to 800 °C, and kept at that temperature for 3 days to obtain homogeneous monophosphides. The powder diagrams of the products experimentally obtained contained only the reflections of HfP and NbP, respectively. For the synthesis of Hf5Nb5Ni3P5 a stoichiometric mixture of 5 mmol of HfP, 5 mmol of Nb, and 3 mmol of M (FISHER, purified powder) was thoroughly mixed and then cold-pressed to a pellet. This pellet was arc-melted twice on a water-cooled copper hearth under an Argon flow with inversion of the sample after the first melt. Single crystals were obtained after annealing in a tungsten crucible in an induction furnace at 1350 °C under dynamic vacuum (10-6 bar) for 5 h. The Guinier diffraction pattern showed only the reflections of Hf5Nb5Ni3P5. All our attempts to substitute any of the- metals of Hf5Nb5Ni3P5 failed. We arc-melted and annealed different samples of the initial compositions (M1)x(M2)10-xM′3P5 with M1 ) Zr, Hf; M2 ) Nb, Ta; M′ ) Co, Ni; and x ) 0, 2.5, 4, 5, 6, 7.5, 10. The new structure discussed here occurred only in the Hf-Nb-Ni-P system. The powder diagrams of the samples with the starting compositions “Hf4Nb6Ni3P5” and “Hf6Nb4Ni3P5” consisted mainly of the reflections of the HfxNb10-xNi3P5 phase; only a few other, weak unidentified reflections were observed. Indexing of the three Guinier patterns of the samples with the initial compositions “Hf4Nb6Ni3P5”, Hf5Nb5Ni3P5, and “Hf6Nb4Ni3P5” yielded the different lattice constants shown in Table 1. As expected on the basis of the different Pauling single bond radii for Hf (1.442 Å) and Nb (1.342 Å),24 the cell dimensions increase significantly with increasing Hf content. It is concluded that Hf5Nb5Ni3P5 is not a line compound, but that the phase range of HfxNb10-xNi3P5 is rather small with 4 < x < 6. Electron Dispersive Spectroscopy (EDS). To check for adventitious impurities, i.e. W from the crucible or Cu from the Cu hearth, Hf5Nb5Ni3P5 crystals were scanned with a JSM-840A scanning microscope (JEOL). No impurities were found, and in all cases, all four elements, Hf, Nb, Ni, and P, were present. According to the standardless EDS analysis, the composition of the crystals was determined to be homogeneous within the range of the standard deviations of about (10% for each element, which can be explained by the different orientations of the scanned surfaces. Structure Determination. A needle-like single crystal isolated from a sample with the starting composition Hf5Nb5Ni3P5 was selected for (24) Pauling, L. The Nature the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1948.
Table 2. Crystallographic Data for Hf5Nb5Ni3P5 empirical formula mol wt temp of data collection cryst dimens space group unit cell dimens no. of formula units calcd density abs coeff F(000) scan mode, scan width scan speed range of 2θ no. of measured reflcns no. of independent reflcns no. of obsd reflcns (I > 3σ(I)) no. of parameters refined R(F2), Rw(F2), goodness of fit (GOF) extinction coeff max, min peak in final diff map abs corr min, max transmission
Hf4.96(4)Nb5.04(4)Ni3P5 1687.95 g/mol 295 K 0.02 mm × 0.002 mm × 0.002 mm P6h2m (No. 189) a ) 9.561(3) Å, c ) 3.5237(5) Å V ) 279.0(2) Å3 1 10.047 g/cm3 56.23 mm-1 724 ω-θ, (1.21 + 0.34 tan θ)° 4.0 deg/min (in ω, 3 rescans) 4-70° 2616 511 (Rint ) 0.071) 267 25 0.045, 0.054, 1.02 0.74061 × 10-6 6.42 e-/Å3, -4.24 e-/Å3 Ψ scan 0.45-0.99
data collection and mounted on a glass fiber. The crystal quality was checked with a Laue photograph, using a Weissenberg camera. The data were collected on a Rigaku AFC6R diffractometer equipped with graphite monochromated Mo KR radiation and a 12KW rotating anode. The orientation matrix for the data collection and the preliminary cell constants were obtained from a least-squares refinement using 17 carefully centered reflections in the range 17.4° < 2θ < 47.5°. The ideal hexagonal lattice constants were refined to a ) 9.561(4) Å, c ) 3.5237(5) Å, and V ) 279.0(2) Å3. The good agreement with the cell dimensions obtained from the bulk sample confirms that the crystal is representative of the whole sample. Four octants were measured with the ω-2θ technique to a maximum 2θ value of 70° and later merged. The intensities of three standard reflections, measured after every 100 reflections, remained constant throughout data collection. The data were corrected for Lorentz and polarization effects. No systematic extinctions were found, and the distribution of the normalized structure factors pointed to a noncentrosymmetric space group. Thus, the highest reasonable symmetry is that of the space group P6mm. However, it turned out that the structure model obtained in this space group could be refined to reasonable residual factors, but one position (6d) was refined best as having a 1:1 mixed occupancy of nickel and phosphorus. Since all atoms were located either on z ) 0 or z ) 1/2, that structure model could also be refined in the space group P6/mmm. Reducing the symmetry to P6h2m occurs with two independent 3g positions instead of the 6d site, and yielded a structure that refined with fully occupied Ni and P sites. It is concluded that P6h2m is the correct space group. The structure was solved by direct methods, using the SHELXS-86 program package.25 Six peaks with reasonable distances to each other (>2.1 Å) were taken from the e-map. The three strongest peaks all had different heights, and were thus considered as 100% occupied mixed Hf/Nb sites. According to their heights and interatomic distances, the three other peaks were identified as one Ni and two P positions. Final refinements of this model, using the TEXSAN program package,26 yielded uniform temperature factors for all six positions. The occupancy factor was determined to be 100% Hf of the first metal site (M1), 59.5(4)% Hf and 40.5(4)% Nb of M2, and 13.1(6)% Hf and 86.9(6)% Nb of M3. This results in the empirical formula Hf4.96(4)Nb5.04(4)Ni3P5, which is, within the standard deviations, equal to the starting composition. The crystallographic data for Hf5Nb5Ni3P5 are summarized in Table 2, and the atomic positions, equivalent temperature factors, occupancy factors, and the anisotropic temperature factors, are contained in the Supporting Information. (25) Sheldrick, G. M. SHEIXS-86, Universita¨t Go¨ttingen: Germany, 1986. (26) TEXSAN: Single Crystal Structure Analysis Software, Version 5.0; Molecular Structure Corporation: The Woodlands, TX, 1989.
12826 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Kleinke and Franzen
Figure 1. Projection of the structure of Hf5Nb5Ni3P5 along [001]: large, black circles, M1; large, heavily dotted circles, M2; large, medium dotted circles, M3; medium, white circles, P; small, black circles, Ni atoms. Only M-M and Ni-P bonds are shown; thicker lines, M-M bonds
